Conventional automatic transmissions use a set of gears that provide a given number of ratios. The transmission shifts gears to provide the most appropriate ratio for a given situation. Lowest gears are used for the start of a drive, middle gears are used for acceleration and passing, and higher gears are used for fuel-efficient driving. A constantly or continuously variable speed transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of effective gear ratios. This differs from other mechanical transmissions that offer a fixed number of gear ratios.
Some CVTs use large belts such as oversized fan belts, while other CVTs use metal mesh or casings to help strengthen them. However, these types of CVTs are not sturdy and are inordinately loud.